SciCafe: Trilobite Takedown

SciCafe: Trilobite Takedown – Video Transcript 

Melanie Hopkins (Assistant Curator, Division of Paleontology): The Earth is about four-and-a-half billion years old. And for the first four billion years of Earth’s history, there really isn’t a lot going on. And that’s what’s shown here in this picture. This is a picture describing the evolution of life on Earth, at least as far as the fossil record is concerned. So, that doesn’t—in those first four billion years of Earth’s history, it doesn’t mean that there’s no evidence. It’s just that there’s indirect evidence.

There are some organic molecules that can be linked to some modern groups. There’s also chemistry in the rocks that tells us that the amount of oxygen in the atmosphere was increasing, and that that might be associated with the evolution of photosynthetic organisms in the ocean, particularly algae. But it’s not until about 800 million years ago that we finally start to see little bits of skeletal material in the fossil record, things like little plates, little spicules, little bits of shells. And some of them we can relate to modern organisms, but a lot of them are still really enigmatic.

And then all of a sudden, and when I say all of a sudden, I’m talking about over a period of about 20 or 30 million years, but compared to four billion years of Earth history before that, it qualifies as sudden, we start to see fossil material, macroscopic fossil material, animals in the fossil record. We can see them with the naked eye. We don’t need microscopes. We can relate a lot of them to modern groups, and almost all animal groups are represented, at least the ones that are readily fossilizable.

The other thing that’s going on is that the numbers of these different animals is increasing. So, this is a graph that shows the increase in the number of genera. Genera are just very closely related groups of species, animal species, in this case. The number of genera that’s increasing during this period, during the Cambrian, this period of about 530 million years to 500 million years ago. And like I said, we can recognize these things. We can relate them to modern groups, so there are things like Gogia. This is an early echinoderm. Echinoderms include things like sea urchins and starfish and crinoids. There are really early mollusks. Mollusks include clams and snails and squids. There are brachiopods. And brachiopods look superficially like clams, but they’re actually very distantly related, and their internal anatomy is completely different, but there they are, in the Cambrian. And they’re still around today.

We also even have some early chordates. Chordates is the group that include vertebrates, things like fish and birds and mammals. And then, of course, we have arthropods. Living arthropods today include things like crustaceans, shrimp, lobsters, spiders, insects, scorpions. But the first convincing arthropod in the fossil record is the trilobite. And you can see that the earliest trilobites first appear at the start of this diversification period. Trilobites are also make up a very large percentage of the arthropod fossil record, and the reason for this is that this exoskeleton, the part that you can see in the picture here, was really heavily biomineralized. What that means is that the animal was putting a lot of minerals into its exoskeleton; in this case, calcium carbonate.

It also means that the fossil itself, the organism itself after it died, was less likely to break after it died and was also less likely to decay after it died. And so, it had a much higher chance of making it into the fossil record. In fact, such a high chance of making it into the fossil record that we find them all over the world. So, this is a data set that I downloaded from the paleobiology database. This is an international community effort to take fossil occurrences from field notes and from the literature and put them into a database that we can use to answer really broad questions. And so far, about 50,000 trilobite occurrences have been entered. And that’s what’s shown here. They’re color-coded by their age.

Now just because we find a trilobite fossil way up north in Greenland doesn’t mean that when that particular trilobite was alive that it actually lived really near the poles. We know this because the continental plates have moved over time. But we can model that change, and so we can make maps of where the continents were hundreds of millions of years ago. And then we can take these fossil occurrences and project them back onto those maps. So, this is what the Earth looked like in the Cambrian, around 520 million years ago. The continent over here that I’ve labeled North America is the early North American continent. North America’s really old. Over time, other continental plates have slammed into it. We’ve got the rise of the Appalachians, the rise of the Rocky Mountains, but the continent itself has been around for at least 600 million years.

In contrast, this other continent that I’ve labeled Siberia is indeed Siberia. It’s what’s now in Eastern Russia. This other continent, Northern Europe, is made mostly up of what’s now Scandinavia and Baltica. And then there was a really large continent near the pole that we’ve called Gondwana, and this includes much of South America and Africa, Australia and Antarctica as well. So, here’s the projection of all of the trilobite fossil occurrences in the Cambrian in their paleo-locality. And you can see that back in the Cambrian they also were distributed all across the world. They were around the equator. They showed up in polar regions. In addition to this, they lived in all different marine environments: deep water, shallow water, high-nutrient environments, low-nutrient environments.

And they were also really abundant. They made up a major component of these communities at the time, and we can see this actually quite clearly if we compare the trilobite occurrences during the Cambrian to all of the other fossil occurrences. Both at the local level and at the global level, they were very abundant and diverse. This is just a selection of Cambrian trilobites from around the world. And this diversity that they already were starting to accumulate in the Cambrian, they carried with them through much of their evolutionary history.

So, the big question, then, is why is—how did this diversification happen? And I’m going to—for the rest of my talk, I’m going to propose a mechanism for how this could have occurred in trilobites, but before I do that, I need to give you a little more information about trilobite anatomy and trilobite growth. I’m going to do that using Elrathia kingi. This might be a very familiar looking trilobite to some of you. It’s found in western Utah, but literally millions of specimens have been collected. It shows up in rock shops, it shows up in educational kits for schools. If you have a trilobite fossil at home, I wouldn’t be surprised to find out this was what you had. And I’m going to use it here today to give you just a basic overview of the different parts of the exoskeleton.

So, in the drawing on your right, the cephalon, which I’ve colored in blue, this is the head shield. This is basically the business end of the trilobite. This feature in the middle, this sort of globular feature in the middle, is the glabella. That was on top of the stomach, and underneath that was the mouth on the underside of the body. And then these two little lobes on the side cover up the eyes or on top of the eyes. And the eyes themselves were compound eyes, like we see in insects. But each one of those lenses was actually mineralized. They were little crystals.

Behind the head is a series of thoracic segments. These elongate segments are actually jointed, so the trilobite can move around them. And each one of these thoracic segments had a pair of legs and a pair of gills associated with it. And you can see that here. So, now we're looking at the underside of a trilobite. The head is over here on the left with the two antennae sticking out, and you can see the little lobe of one of the eyes poking out at the top. There are some legs underneath the head, and then there are also legs associated with each one of those segments. This is really rare, by the way. We have about two—over 20,000 named trilobite species, and we know about—we have preserved appendages, either antennae or walking legs, for only about 20 of those species.

So, I mentioned earlier that the trilobite exoskeleton was really heavily biomineralized. It was really hard. And like all arthropods, in order to grow, the trilobite had to get rid of this really hard exoskeleton in order to get bigger. And it did this by molting and essentially by allowing the different parts of the exoskeleton to break apart from one another. Some crustaceans do this by actually reabsorbing the calcium carbonate along suture lines, and this lets those parts break apart. In the case of this trilobite spices and many others, the head broke apart along these really thick lines that I’ve drawn here. And so, in so doing, it actually enabled the trilobite to crawl out of the exoskeleton and leave it behind.

So, after this thing molts, it’s left behind an exoskeleton that has a really high preservation potential. And so, what this means is that as a trilobite was growing, it was leaving behind all of these exoskeletons. They all had the potential to end up in the fossil record, and many of them did. So, we actually know a lot about trilobite development. This is an example of trilobite growth for a shumardii xalapensis. This is a really classic example. This was first described back in the 1800s, shows up in textbooks and papers. And as this winds through and shows you some of the stages during development of this trilobite, you’re probably noticing two things.

One, the number of segments is increasing. The other thing is that there’s this one segment that’s got a really big spine hanging off of it, and that seems to be moving forward in the body during development. The reason it looks like that is because every time the trilobite molted, it added a new segment at the very back of the body and released a segment from the pygidium, the pink part, the tail, into the middle thorax. So, if you look here, from the very early stage labeled zero to m-1, it’s that very front segment in the pygidium, the pink part, that’s being released into the thorax. And it looks like it’s moving forward, because another segment is being added at the back. And then, when you get a feature that’s associated with just one segment, like the spine, you can actually watch it travel through the pygidium as more segments are added behind, until it gets released into the thorax and then continues to stay, in this case, at the fourth segment as more segments are added.

Now at some point, the trilobite stopped adding segments. It continued to grow and molt, but it didn’t add any more segments to the very end, and it didn’t release any more into the thorax. And this transition from juvenile to adult was associated with a lot of different other changes including decrease in growth rates in some cases and also a less general change in some other features. We have enough data from enough species now that we can actually model this process. And so, what I’ve done here is I’ve modeled growth for Elrathia kingi. And there are a lot of different parameters that go into this model. There are parameters that determine the original size, the size of the larval form when it first starts to molt and add segments. There’s a parameter that determines the number of segments that are added, and how many will be in the adult trilobite, the terminal number of segments. And there are also a bunch of parameters that determine the growth rates of all of these different parts and allow for those rates to change during development.

Now that we have this model, what this means is that we can play around with those parameters and try to make trilobites of different sizes and different relative proportions. So, here what I’ve done is I’ve—the only thing I’ve changed in the model is to increase the number of terminal segments. So, before, the real Elrathia kingi has 13 segments in the thorax. In the model over here, I’ve let the trilobite grow until it was 20 segments. You can see that the final trilobite gets bigger. Not a lot bigger, but a little bigger over the same number of molts. If instead we change some of those growth rates, particularly the growth rates associated with the thorax and the pygidium, we can get a trilobite that is the same number of segments, 13, but is much bigger. We can also get a trilobite that’s got a really big head. And I’m guessing you can all figure out how to make a trilobite with a really big head. You just let the growth rate for the head increase much more than anything else.

And we can play with parameters simultaneously. So, here I’ve decreased the number of segments in the body, the number of total, final segments in the body, and increased the growth rates, in this case, of the pygidium, primarily during the adult stage. And you can see that the cephalon, the head in blue, the thorax in white, and the pygidium in red are all about the same size now. And indeed, we see trilobites that look this way, that have these proportions. So, over here, this is a Cryptolithus, a species of Cryptolithus, and you can see that the head, which would be about there, is about half the size of the total length of the body. And then this species of Isotelus, the head, the thorax, and the tail are all about the same size. So, this is cool. Or I think this is cool. But you can see these trilobites look very different.

So, while this model is letting us play around with the sizes of the trilobite and the relative proportions of these things, it still hasn’t given us any insight into why these guys look so different. And they can look really different, and they also have features on these different parts that look really different. They can have small eyes and big eyes. They can have long spines, they can have short spines, no spines at all. But I would argue that the answer actually is in that growth model, and the reason is because as all of these different parts are growing, they’re also changing shape. So, this is an example of a trilobite from southwest China. The adult is shown in the picture of the actual fossil, and then all of these line drawings show different trilobite stages during development, starting with A in the left-hand corner and working across.

And you’ll probably notice right off the bat that the spines get shorter as the trilobite gets bigger. But there’s also some more complex morphological changes happening, too, and one of them’s related to the shape of this eye complex. So, the eye ridge, which is shown in pink, in the very early stages, in the very youngest juveniles, is not very strongly curved, and the eye itself, where all the lenses are, you can barely see it here, actually. It’s shown in red, but it’s just a little, little bit on the edge. And then as a trilobite grows, this complex gets much more curved. The eye itself also gets more curved and gets bigger, and it moves down the body, or it moves further back along the body relative to where it was before.

What this means is that if you have all of these different parts that are changing shape but also changing at different rates, if you start to tinker with those growth rates, you can start getting trilobites that look different from one another. I’ve drawn that very schematically here, and this doesn’t look like a trilobite at all, but that’s the point. That’s on purpose. And the reason is because this is a mechanism that I think can apply to any animal that has different parts that grow relatively independent from one another at different rates. So, here in my make-believe animal, it’s made up of three parts. One of them is this yellow triangle, and through development it gets bigger relative to its width or longer relative to its width. The part that’s represented by the blue trapezoid also gets longer relative to its width.

If we change those rates, we still let the yellow triangle get longer relative to its width, but at a much faster rate, and we still let the blue trapezoid get longer relative to its width, but at a much slower rate. We end up with things that look different from one another, and this is just a very simple example. We’ve been able to start documenting this quantitatively in some trilobite species that are closely related to one another. And I’ve also actually just recently was in conversation with another curator here at the museum who’s been working on ammonites who’s also looking at how new morphologies arise from changes in rates of development.

And so, with that, I would just end by saying there’s still a lot of diversity out there that we don't know how it originated. We don’t know how the traits originated or how new adaptations arose, but I think by learning more about development and growth, we can start getting closer to those answers. Thank you.